Spacecraft May Surf Magnetic Fields

Maggie McKee writes "Future spacecraft may surf the magnetic fields of Earth and other planets, taking previously unfeasible routes around the solar system, according to a proposal funded by NASA's Institute for Advanced Concepts. The electrically charged craft would not need rockets or propellant of any kind. To get charged up, the spacecraft could either fire a beam of charged particles into space or simply allow a radioactive isotope to emit charged particles. Because long, thin filaments have large surface areas for holding charge, the spacecraft might look like Einstein's head — with wiry filaments sticking out at all angles — or a weird space 'stocking.'"

Comments

Re:Last I Checked

[headkase]

The Sun's magnetospere is bigger than the orbit of Pluto if I remember correctly. And it doesn't have to be exclusive, you could use the magnetic "sock" for a stage of the journey and maybe propellents in another stage of the same voyage and then you short range teleport for the last 5000km ;). It's just another useful tool to employ from a mission planning perspective.

Umm... capacitance of the ball made of filaments?

[Alex Belits]

But the amount of charge held by a sphere at a given voltage, a quantity known as its capacitance, is not very large. Long, thin filaments, on the other hand, have a lot of charge-holding surface area, so one possible design involves many filaments attached to the spacecraft. The setup would have a rather comical look - because of the static charge, the filaments would stick out in all directions, like newly brushed dry hair.

Where did those people study Physics? It doesn't work that way.

The only surface area that matters is an equipotential surface, so in the case of "filaments sticking in all directions" it will be a roughly spherical surface formed by the ends of filaments. Within this sphere there is almost no electric field -- filaments can be seen as a kind of lightning rods, except there is no lightning because they are in vacuum. So at best they will have a larger sphere, at worst a cigar or other shape with less surface area. If one has to build a large but light sphere, he can make it out of the wire mesh -- in vacuum it won't discharge like it would in the air, where those spheres have to be smooth. Filaments or spikes can be useful for acceleration of charged particles.

As for usefulness of the whole thing, I guess, you can use this for steering the spaceship, however the analogy to surfing is very poor. Surfboards can accelerate by absorbing the energy of waves moving from deep to shallow water. This thing flies through a stable magnetic field, steering by changing its electric charge. A better analogy would be a sailboat changing tacks, with gravity acting as a wind and magnetic field as water resistance.

Remember Force=Velocity x B (magnetic field)

[viking80]

Anyone who remember physics 101 will recall the following:
A magnetic field will create a force normal to the velocity of a charged object. So
1. The force on any charge will be normal to the velocity, so it will move around in circles. This includes the charged spacecraft. (The field here is not homogeneous, but still, no gain in energy)
2. A current loop(i.e. a moving charge) can change the energy or be accelerated in a magnetic field.

(2) is and has been used since the first satellites were orbited to do orbital corrections, and are well understood and used. It can allow the life of the satellite to be expanded by many years.

Why would this craft not just use a long current carrying wire instead.

Re:Umm... capacitance of the ball made of filament

[Alex Belits]

Corona in a gas is easy to produce because gas can be ionized by strong electric field along the spike or fiber, giving you a lot of charged particles to carry your discharge current. In vacuum you have nothing but those electrons that are already in metal -- you can emit them if you are negatively charged, and can't emit anything if you are positively charged (think of it as a giant diode tube). Even if you emit electrons, you have to move very fast so magnetic field will divert them away from you, and once you are sufficiently positively charged, emission will stop.

Most likely at best this will produce a cloud of electrons following a positively charged spaceship, so forces that magnetic field applies to both will almost completely compensate each other. With radioactive source of charged particles (positively or negatively charged), or electron cannon you can produce more charge on the spacecraft, and probably it can be combined with ion engine that produces charged particles anyway.

Re:why wires don't work

[frostband]

"induced current was about 9600 volts"

Why do I feel the need to point this out?

I know what he meant, but why did you make me reply? Damn you, EE degree, stop controlling my life!

A few glitches in this idea:

[Ancient_Hacker]

A few glitches in this idea:

• Building up and keeping a substantial charge may be difficult-- the solar wind will tend to get in the way.
• "Steering" is a bit misleading. Assuming you can only shoot out electrons, you're limited to one polarity of charge, so you can only turn in one direction, normal to the magnetic field.
• Most planets have a bit of inclination, so you're going to be turning into an angle to the ecliptic-- not very useful if you want to aim at another planet.
• A sudden solar flare is likely to really mess up the charge, leading to unexpected twists and turns.

electric solar wind sail

[jovius]

There exists another interesting invention around about the same principles. This one however uses solar wind, not magnetic fields, is steerable and able to work itself against the solar wind by adjusting the polarity and charge of the individual wires. They are apparently doing practical research on this right now. A short english description can be found at http://www.electric-sailing.fi/index.html